In one aspect, the invention relates to a catalytic cracking unit. In another aspect, the invention relates to feeding materials to the riser or transfer line in a fluid catalytic cracking unit.
High boiling oils are difficult to catalytically crack to gasoline range product in existing catalytic cracking operations. There are several reasons for this. The deposition of large amounts of coke on the catalyst will frequently bring the unit up to its coke burning capacity. Coke precursors are more abundant in high boiling oils. Coke laydown is also caused by the deposition of metals on the cracking catalyst that increase the coking tendencies of the catalyst. The troublesome metals become concentrated in the high boiling oils. Coke laydown to a large extent is also influenced by poor vaporization of the oil prior to contact with the catalyst. High boiling oils are difficult to vaporize. Poor mixing between the cracking catalyst and oil feedstock also contributes to coke laydown on the catalyst, as poor mixing can lead to localized high catalyst:oil ratios and overcracking.
Heavy oils include heavy gas oils which generally boil from about 600.degree. F. to 1200.degree. F., and components such as topped crudes and residuum which frequently have an initial boiling point in excess of 850.degree. F. and an end boiling point in excess of 1200.degree. F. Generally speaking, heavy oils will have an initial boiling point in excess of 500.degree. F. and a 90% overhead point in excess of 1000.degree. F. Heavy gas oils and residuums are especially difficult to crack to valuable products because their boiling point makes satisfactory vaporization very difficult, their viscosity complicates handling and further complicates vaporization, metal contaminant concentration is usually quite high, the hydrogen:carbon ratio is quite low and the concentration of carbon producing components such as polycyclic aromatics, asphaltenes and the like is very high. Feeds which contain components which have a boiling point in excess of 1050.degree. F.+ are generally considered to be very poor fluid catalytic cracking feeds due to poor conversion to gasoline and lighter components, high coke production and excessive temperature levels in the regenerator.
Heavy oils can be successfully cracked to desirable products where they have been vaporized prior to contact with the catalyst and the catalyst:oil ratio is carefully controlled. With conventional feeds, vaporization is achieved by radiant energy transfer from the hot cracking catalyst to the feed droplets. This type of vaporization mechanism is satisfactory for oils boiling below thermal cracking temperatures which commence at about 850.degree. F. For heavy oils, however, vaporization by radiant energy transfer is unsatisfactory due to the onset of thermal cracking and coke formation prior to complete vaporization. Coke laydown is worsened where liquid oil strikes the hot catalyst particles. It would be clearly desirable to provide an apparatus and process to mitigate contact between hot catalyst and liquid oil feed in a catalytic cracking unit.
It has been proposed to atomize the cracking feedstock in an atomization chamber prior to admixing the atomized feedstock with cracking catalyst. In one arrangement for accomplishing this, the cracking catalyst is flowed annularly around the atomization zone. Heat penetration from the cracking catalyst through the walls of the atomization zone could cause undesirable coke formation on the walls of the atomization zone. An apparatus to provide for cooling the walls defining the atomization zone and a method for cooling the walls would clearly be very desirable.